Chlorine is an important bulk product in the chemical industry (Moussallem, I., et al., J Appl Electrochem (2008) 38 (9), 1177). It is used to manufacture isocyanates, and chlorinated/fluorinated hydrocarbons; hydrogen chloride is a by-product of these processes. Since the hydrogen chloride market is oversaturated and there is significant demand for chlorine, chlorine recycling from hydrogen chloride becomes increasingly more desirable. A number of commercial processes have been developed to convert hydrogen chloride into usable chlorine gas.
Two different processes are used to convert aqueous hydrochloric acid or gaseous hydrogen chloride into chlorine: a thermal catalytic oxidation and an electrochemical process. Commercial catalytic oxidation processes are based on the Deacon reaction and known as the “Shell-Chlor”, the “Kel-Chlor”, and the “MT-Chlor” processes (Wattimena, F., and Sachtler, W. M. H., Catalyst Research for the Shell Chlorine Process. In Studies in Surface Science and Catalysis, Seiyama, T., and Tanabe, K., (eds.) Elsevier (1981), Vol. 7, Part B, pp 816). The Deacon process uses a fixed bed or fluidized bed containing catalysts (Perez-Ramirez, J., et al., Energy and Environmental Science (2011) 4 (12), 4786). Various catalysts such as copper, ruthenium oxide, rare earth compounds, various forms of nitrogen oxide, and chromium oxide have been developed by different companies for the reaction of equation (1).

Although a low temperature Deacon process is relatively energy-efficient, it has the disadvantages of low conversion, high capital cost, corrosion, high catalyst cost, and short catalyst lifetime (Mortensen, M., et al., Chemical Engineering Science (1996) 51 (10), 2031). Moreover, these Deacon processes are operated at elevated pressures and temperatures of 250° C. and above and are most economical at large scale.
There are two electrochemical processes for converting hydrochloric acid or gaseous hydrogen chloride into chlorine (Kuwertz, R., et al., Electrochemistry Communications (2013) 34, 320; Martinez, I. G., et al., Electrochimica Acta (2014) 123 (0), 387). In the first process, a hydrogen evolution reaction (HER) is used at the cathode, and in the overall reaction, hydrogen chloride is split into chlorine and hydrogen (equation 2). In the second process, an oxygen depolarized cathode (ODC) is used, and in the overall reaction, the oxidation of hydrogen chloride generates chlorine and water (equation 3).HCl—Cl2+H2  (2)4HCl+O2—2Cl2+2H2O  (3)
The reversible cell voltage theoretically can be significantly lower when an ODC is used instead of a HER at the cathode.
The electrochemical conversion processes can also be distinguished according to whether aqueous hydrochloric acid or gaseous hydrogen chloride is used as shown in Table 1.
TABLE 1Comparison of known electrolysis processes for chlorine production from HClBayer-Uhde-Bayer-UhdenoraDupont-DenoraHoechst ProcessProcessProcessCurrent density4410(kA/m2)Unit-cell 21.351.7voltage (V)Energy1,50010201,250consumption(kWh/tCl2)AnodeGraphiteTi/RuO2 (DSA)RuO2HCl-formDissolved inDissolved inGaseouswaterwaterCathodeGraphiteRhxSyPt/CCathode HERODCHERreactionSeparatorPVCNafionNafionTemperature 60-90° C.60° C.80° C.(° C.)HER: Hydrogen evolution reaction;ODC: Oxygen depolarization cathode.
The electrochemical process for splitting aqueous hydrochloric acid into chlorine and hydrogen is called the Bayer-Hoechst-Uhde process. The aqueous solution used for the Bayer-Hoechst-Uhde process is 22 wt. % hydrochloric acid and operates at a current density of 4 kA/m2 at a cell voltage of 2 V. This results in an energy consumption of 1500 kWh t(Cl2)−1 for a standard diaphragm cell. The process using an ODC in a membrane cell to convert aqueous hydrochloric acid into chlorine is called the Bayer-Uhdenora process. In this process, a gas diffusion electrode is used for the ODC, while a dimensionally stable anode (DSA) is used for chlorine evolution. The electrolyzer can be operated at a current density of 4 kA/m2 at a cell voltage of 1.35 V, and so the energy consumption can be reduced to 1020 kWh t(Cl2)−1 for the membrane cell with the ODC.
The Bayer-Hoechst-Uhde and Bayer-Uhdenora processes involve aqueous hydrochloric acid feed to the anode. The similarity of the reversible potentials for the evolution of oxygen and chlorine in an aqueous solution combined with the mass transport limitations in a liquid causes the production of oxygen in the anode in an aqueous hydrochloric acid electrolysis process. The result is a decrease in current efficiency and corrosion of cell components.
A gaseous hydrogen chloride electrolysis process avoids this problem and operates at high current density. Another advantage of using gaseous hydrogen chloride is that the theoretical chlorine evolution potential is lowered by at least 0.3 V (0.99 V for gaseous hydrogen chloride versus 1.36 V for aqueous hydrochloric acid at standard conditions). This results in a cell voltage reduction of 0.3 V. The first proof of principle of an electrolysis of gaseous hydrogen chloride was demonstrated by DuPont (Eames, D. J., and Newman, J., Journal of the Electrochemical Society (1995) 142 (11), 3619; U.S. Pat. No. 5,411,641). This process for splitting gaseous hydrogen chloride into chlorine and hydrogen was conducted in a fuel cell type reactor. The reported cell voltage was 1.7 V at a current density of 10 kA/m2 and resulted in an energy consumption of 1250 kWh t(Cl2)−1.
Further improvements of the gaseous hydrogen chloride electrolysis process employed an ODC at the cathode. Vidaković-Koch et al. indicate that the limiting step in the gaseous hydrogen chloride electrolysis with an ODC is the slow oxygen reduction reaction (ORR) kinetics (Martinez, I. G., et al., Electrochimica Acta (2014) 123 (0), 387). At a practical current density of 4 kA/m2, the overpotential for the oxidation of hydrogen chloride gas (HClOR) is about 0.09 V but the overpotential for the ORR is about 0.8 V even with a platinum catalyst which is considered the best electrocatalyst for the ORR. However, platinum does not have sufficient stability in the highly corrosive hydrochloric acid electrolyte and chloride ions can strongly adsorb on the platinum surface, leading to low activity of the ORR (Maljusch, A., et al., Analytical Chemistry (2010) 82 (5), 1890; Ziegelbauer, J. M., et al., Electrochimica Acta (2007) 52 (21), 6282). A rhodium-based material, specifically RhxSy, (Jin, C., et al., ChemSusChem (2011) 4 (7), 927; Gullá, A. F., et al., Applied Catalysis A: General (2007) 326 (2), 227) was applied as an oxygen reduction catalyst for a commercial aqueous hydrochloric acid electrolysis with an ODC process. However, rhodium is also an expensive precious metal which would increase the electrolysis cell cost.